Transform Domain Based Channel Estimation for 3GPP/LTE Systems

Abstract

Orthogonal frequency division multiplexing (OFDM) is now well known as a powefull modulation scheme for high data rate wireless communications owing to its many advantages, notably its high spectral efficiency, mitigation of intersymbol interference (ISI), robustness to frequency selective fading environment, as well as the feasibility of low cost transceivers [1]. On the other hand multiple input multiple output (MIMO) systems can also be efficiently used in order to increase diversity and improve performance of wireless systems [2] [3] [4]. Moreover, as OFDM allows a frequency selective channel to be considered as flat on each subcarrier, MIMO and OFDM techniques can be well combined. Therefore, MIMO-ODFM systems are now largely considered in the new generation of standards for wireless transmissions, such as 3GPP/LTE [5] [6]. In most MIMO-OFDM systems, channel estimation is required at the receiver side for all sub-carriers between each antenna link. Moreover, since radio channels are frequency selective and time-dependent channels, a dynamic channel estimation becomes necessary. For coherent MIMO-OFDM systems, channel estimation relies on training sequences adapted to the MIMO configuration and the channel characteristics [7] and based on OFDM channel estimation with pilot insertion, for which different techniques can be applied: preamble method and comb-type pilot method. In order to estimate the channel of an OFDM systems, one’s first apply least square (LS) algorithm to estimate the channel on the pilot tones in the frequency domain. A second step can be performed to improve the quality of the estimation and provide interpolation to find estimates on all subcarriers. In a classical way, this second step is performed in the frequency domain. An alternative is to perform this second step by applying treatment in a transform domain, that can be reached after a discrete Fourier transform (DFT) or a discrete cosine transform (DCT), and called transform domain channel estimation (TD-CE). The DFT based method is considered as a promising method because it can provide very good results by significantly reducing the noise on the estimated channel coefficients [8]. However, some performance degradations may occur when the number of OFDM inverse fast fourier transform (IFFT) size is different from the number of modulated subcarriers [8]. This problem called ”border effect” phenomenon is due to the insertion of null carriers at the spectrum extremities (virtual carriers) to limit interference with the adjacent channels, and can be encountered in most of multicarrier systems.